Disks using the Apple Partition Map are divided into logical blocks, with 512 bytes usually belonging to each block. The first block, Block 0, contains an Apple-specific data structure called “Driver Descriptor Map” for the Macintosh Toolbox ROM to load driver updates and patches before loading from a MFS or HFS partition.[2] Because APM allows 32 bits worth of logical blocks, the historical size of an APM formatted disk using small blocks[3] is limited to 2 TiB.[4]

The Apple Partition Map maps out all space used (including the map) and unused (free space) on disk, unlike the minimal x86 master boot record that only accounts for used non-map partitions. This means that every block on the disk (with the exception of the first block, Block 0) belongs to a partition.

Some hybrid discs contain both an ISO 9660 primary volume descriptor and an Apple Partition Map, thus allowing the disc to work on different types of computers, including Apple systems.

Intel-based models that came with Mac OS X Tiger (10.4) or Leopard (10.5) preinstalled had to be able to boot from both APM and GUID disks due to the installation media for these universal versions of Mac OS X, which are APM partitioned in order to remain compatible with PowerPC-based systems.[8] However, the installation of OS X on an Intel-based Mac demands a GUID partitioned disk or will refuse to continue, the same way installation on a PowerPC-based system will demand an APM partitioned destination volume. Cloning an already installed OS X to an APM partition on Intel systems will remain bootable even on 2011 Intel-based Macs. Despite this apparent APM support, Apple never officially supported booting from an internal APM disk on an Intel-based system. The one exception for a universal version of Mac OS X (Tiger or Leopard) is an official Apple document describing how to set up a dual bootable external APM disk for use with PowerPC and Intel.[9]

Each entry of the partition table is the size of one data block, which is normally 512 bytes.[1][10] Each partition entry on the table is the size of one block or sector of data. Because the partition table itself is also a partition, the size of this first partition limits the number of entries to the partition table itself.

The normal case is that 64 sectors (64 × 512 = 32 KB) are used by the Apple Partition Map: one block for the Driver Descriptor Map as Block 0, one block for the partition table itself and 62 blocks for a maximum of 62 data partitions.[11]

Each partition entry includes the starting sector and the size, but also a name, a type, a position of the data area and possible boot code. It also includes the total number of partitions in that partition table.[12] This ensures that, after reading the first partition table entry, the firmware is aware of how many blocks more to read from the media in order to have processed every partition table entry. All entries are in big-endian byte-order.[citation needed]

This boot partition is used by Mac OS X on New World Macs (Open Firmware 3.0 and greater) when the file system on the main partition is not supported by Open Firmware, like in a software RAID configuration or when using a HFS+ case-sensitive or a UFS file system. It contains BootX on an HFS filesystem.

This partition contains a HFS+ volume without a HFS wrapper. HFSX was introduced with Mac OS X 10.3 and is only used in special cases, like case sensitive HFS+. HFSX is the standard partition type on Intel-based Macs (which use GUID instead of APM).

Apple_Loader

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SecondaryLoader

Like Apple_Boot but on Old World Macs, it is used when Mac OS X is installed on a file system not readable by Open Firmware. This partition does not contain a filesystem—instead it contains the BootXmachine code in XCOFF format. This partition type was discontinued with Mac OS X 10.3.

This identifier marks a Mac OS X partition used in a software RAID configuration. It normally contains the same filesystems a regular Mac OS X installation would have, like HFS/HFS+ or UFS. The separate boot partition Apple_Boot is mandatory.

Originally introduced for A/UX (Apple Unix operating system based on System V Release 2, hence SVR2) on the 68k, it was later reused for MkLinux which used the Extended file system. It is the standard partition identifier for many Unix-like operating systems, including Linux and NetBSD. It may contain any file system suitable for the installed operating system. If bootable, a file system that can be read by the Open Firmware bootloader from Apple_Bootstrap (e.g. yaboot) must be used.